Pattern transfer is a lithographic process of utmost importance in the fabrication of VLSI circuits, video discs, far infrared couplers, micro diagnostic devices for internal surgery, x-ray zone plates, and Josephson Junction devices. There are four major lithography methods in use of fabricating microstructures in the manufacture and packaging of very large scale integrated circuits. These are ultraviolet (350-430 nm) and deep ultraviolet photolithography (less than 330 nm) using incoherent and coherent light sources, soft x-ray lithography, electrom beam lithography, and ion or proton beam lithography. For obtaining submicron sized features, electron beam lithography is found to be a superior method to optical photolithography, deep ultraviolet photolithography, and even x-ray lithography. Ion beams even exceed electron beams in spatial resolution, but the penetration depth of ion beams is smaller, causing non-uniform resist exposure versus resist depth. Higher energy ion beams are capable of greater resist penetration but they may damage the underlying substrate, resulting in radiation damage and associated loss of reliability of the final fabricated components.
Electron beam lithography enjoys a favored position in research because of its excellent spatial resolution capability. Feature sizes as small as 100 Angstroms in electron sensitive organic resists and even 10 Angstroms in inorganic resists have been achieved. There are basically two types of processes employed in electron beam lithography, namely, serial direct write systems which use programmed deflection of point source electron beams and parallel proximity printing which uses a combination of a wide area electron beam and a wide area electron beam transmission mask. Both of these prior art methods use a thermionically created electron beam system operating in a high vacuum environment. The direct write systems use a tightly focused spot which is moved over the surface of the resist by electromagnetic forces under the control of a computer on which the pattern is pre-programmed. Subsequently, the exposed regions are usually wet developed and sometimes dry developed to reveal the exposed spatial features. The throughput of wafers or die packages per hour, however, is low and the cost is high for direct write systems.
The low throughput problem is overcome to some extent by the use of wide area flood exposure electron beam systems. In the flood exposure system using a photocathode, ultraviolet radiation is used to produce a patterned source of photo-electrons from an ultraviolet transmission mask coupled to a photoemitting surface. The resultant spatially patterned wide area electron beam is subsequently used to spatially expose the resist over a wide area. These parallel print systems are also expensive because they require a high quality ultraviolet transmission mask and also require a high vacuum electron beam system and photoemitting surface. Throughput, however, is improved over direct write systems for both wafers and die packages. An alternative flood exposure system uses a wide area electron transmission mask in close proximity to the resist surface. Electrons transmitted through the mask interact directly with the resist or polymer material on the wafer or die package and bring about molecular weight changes that result in solubility changes. This conventional proximity (1:1) method also requires a further development step, either wet or dry. Also reported in the prior art are projection (n:1) electron beam systems in which a large area electron beam passes through a wide area electron transmitting mask. The patterned electron beam is then passed through a demagnifying magnetic device to reduce the size of the pattern and exposure the resist or polymer material located on the wafer or die package. This conventional projection method also requires a further wet or dry development step. All of the previously described prior art types of flood exposure electron beam systems require a high vacuum ambient environment, a hot filament to supply electrons, and a high voltage to accelerate those electrons. In addition, the wide area electron beam of the latter system described above is usually a modified form of a scanning point source electron microscope having a wide area (mmxmm) beam.
In summary, the prior art electron beam systems for wafer and packaging lithography described above are disadvantageous in that they require high vacuum environments, the throughput of exposed wafers and die packages is low, and they require a further development step to complete the pattern delineation process in the resist or polymer material. These requirements make such systems very expensive. Moreover, the throughput of conventional electron beam lithography will be more limited in the future as wafer sizes increase to six and eight inches in diameter and die sizes increase from 6 mm to 2 cm on a side.